(1) Field of the Invention
The invention relates to a hard-metal cutting tip for use on a drill, particularly on a rock or concrete drill, the cutting tip comprising a drill point for extending through an axis of rotation of the drill, and primary cutting edges that slope downwardly on both sides, each composed of a primary land and a primary flank, the primary flank forming at the primary cutting edge a primary tip clearance angle with a plane of the workpiece perpendicular to the axis of rotation of the drill.
(2) Description of the Prior Art
Known are hard-metal cutting tips for percussion or hammer drills, comprising primary lands and flanks that slope evenly downwardly and primary cutting edges. The drill point is formed by a chisel edge that connects the primary cutting edges. A disadvantage in such cases is that the hard-metal cutting tip is subjected to extreme wear in the outer cutting regions due to the high peripheral speeds. This decreases the service life of the drill. The essential causes of this wear are, on the one hand, the rubbing action of the cutting tip on the working surface, which can cause particles of work material to become welded to the cutting face. These particles break away and roughen the land and the flank. Another cause of wear is diffusion, which, due to the high temperatures produced by the friction, causes metal atoms to migrate from the cutting material into the overflowing chip, thereby weakening the cutting material, which can lead to breakage.
The object of the present invention is to provide a hard-metal cutting tip that has high cutting, feed, bending and breaking strength and wear resistance, especially at its outer regions subjected to high peripheral speeds.
The object is achieved in that the primary lip clearance angle formed at the primary cutting edge decreases with increasing distance from the drill point.
The presence of a primary lip clearance angle that decreases with increasing distance from the drill point protects the primary cutting edges against wear and breakage, especially at the ends of the hard-metal cutting tip. This makes for a longer service life, which is the period of use of the tool
The invention concerns a hard-metal cutting tip for use on a drill, particularly on a rock or concrete drill, comprising a drill point extending through the axis of rotation of the drill, [and] primary cutting edges that slope downwardly on both sides, each composed of a primary land and a primary flank, the primary flank forming at the primary cutting edge a primary lip clearance angle with a plane of the workpiece perpendicular to the axis of rotation of the drill.
Known are hard-metal cutting tips for percussion or hammer drills, comprising primary lands and flanks that slope evenly downwardly and primary cutting edges. The drill point is formed by a chisel edge that connects the primary cutting edges. A disadvantage in such cases is that the hard-metal cutting tip is subjected to extreme wear in the outer cutting regions due to the high peripheral speeds. This decreases the service life of the drill. The essential causes of this wear are, on the one hand, the rubbing action of the cutting tip on the working surface, which can cause particles of work material to become welded to the cutting face. These particles break away and roughen the land and the flank. Another cause of wear is diffusion, which, due to the high temperatures produced by the friction, causes metal atoms to migrate from the cutting material into the outflowing chip, thereby weakening the cutting material, which can lead to breakage.
The object of the present invention is to provide a hard-metal cutting tip that has high cutting, feed, bending and breaking strength and wear resistance, especially at its outer regions subjected to high peripheral speeds.
The object is achieved in that the primary lip clearance angle formed at the primary cutting edge decreases with increasing distance from the drill point.
The presence of a primary lip clearance angle that decreases with increasing distance from the drill point protects the primary cutting edges against wear and breakage, especially at the ends of the hard-metal cutting tip. This makes for a longer service life, which is the period of use of the tool to attainment of a permissible degree of wear. The outer regions of the primary cutting edges, in particular, are severely stressed by the high rotation speeds and the attendant high peripheral speeds. By means of the proposed arrangement, the outer regions can be made to retain their cutting ability for practically as long as the point regions. The drill point region of the hard-metal cutting edge is generally less severely stressed, since the peripheral speed, and thus also the wear, are lower in the central region. The proposed arrangement reduces wear on both the flanks and the lands and the rounding of edges in the outer region of the primary cutting edges. Uniform wear over the full extent of the primary cutting edges ensures that the cutting areas are fully utilized and improves the cost-effectiveness balance. The proposed shape of the primary flank and the primary lip clearance angle can be created as early as in the sintering process and requires no post-processing. For applications involving percussion or hammer drills, the rake is preferably negative and can also be varied along the primary cutting edge in accordance with the lip clearance angle. In this manner, the wedge angle can be kept constant along the primary cutting edge while the lip clearance angle is simultaneously decreased from the drill point to the outer region.
To ensure a uniform cutting, process, it is proposed that the decrease in lip clearance angle be made constant. This prevents the occurrence along the primary cutting edge of stress peaks that increase the likelihood of premature failure of the primary cutting edge at that location.
So that the drill point region is relieved with respect to the primary cutting edge, it is proposed that the decrease in the primary lip clearance angle begin at a defined distance from the drill point. This creates a point region that has a desired width along the primary cutting edge. The width of this region can be determined by the beginning of the decrease in the primary lip clearance angle, and can be varied. This is the region by which the drill head first enters the material, and it centers the drill.
To improve the guidance of the drill in the drillhole, it is proposed that the primary flank transition to a conical transitional face between the primary flank and a narrow-side face, there being a change in the angle of the primary cutting edge at a distance from the drill point. This permits firm guidance of the drill, which is highly desirable especially in the case of very hard materials such as concrete, for example, and reduces the risk of slippage leading to polygonal holes, which can cause excessively large drillholes or even injuries. The proposed arrangement substantially increases the service life of the drill, since wear in this region of the edge can be avoided.
To ensure reliable centering by the centering point and to decrease the force necessary for feed motion, it is proposed that the drill point have a point angle that is smaller than the angle of the primary cutting edges in the vicinity of the transitional face. The same amount of force being applied, this measure results in better penetration of the point into the working surface and simultaneously performs a centering function before the primary cutting edges engage the working surface.
A further improvement of the centering point is achieved by the fact that each primary flank transitions to the drill point by means of a chamfer. This gradual transition from the primary flanks to the drill point reduces the risk of tip breakage and makes the use of relatively hard tips possible. Consequently, even concrete or metal working surfaces can be drilled with long service lives. The centering point makes for a smaller surface area in the core area, the center of the drillhole. This improves drill advance, given the same applied force. The centering point reduces the risk of the drill slipping off center while the hole is being started and furnishes additional guidance. It also prevents the formation of polygonal holes.
To prevent friction during drilling, it is proposed that the small-side face have an outer radius that is adjoined by a flat portion, disposed thereafter in the cutting direction, which broadens with increasing distance from an outer edge. The diagonal elimination of the radius results in effective guidance in the drillhole in the vicinity of the outer edge between the primary flank and the lateral small face, such guidance no longer being necessary in the lower region. The friction surface decreases progressively in that region. This facilitates the cutting motion and decreases the force needed to advance the drillhole. At the same time, because of the decrease in friction during drilling, the material of the cutting tip is not exposed to unnecessarily high temperatures, which again reduces wear of the material due to diffusion or other weakening of the material.